Overload protective device



Jan. 10, 1950 E. H. B. MARTIN OVERLOAD PROTECTIVE DEVICE Filed Dec. 13, 1947 Fi g5.

Irwverw tor: Eric H. B. Martirw His Attovmey Patented Jan. 10, 195i) OFFICE 7 OVERLOAD PROTECTIVE DEVICE Eric H. B. Mutin -Rugby, England, assignor to .General Electric Com New York pany, a corporation of Application December 13, 1947, Serial No. 791,628 In Great Britain July 17, 1947' My invention relates devices for polyphase circuits, and has for its object the provision of a, new and improved device of this character which is simple and inexpensive in construction and rugged and reliable in operation.

According to the present invention, three strips of thermostatic metal are heated by means of three heaters, each heater carrying the currentor a proportion of the current passing in each phase. The free ends of the strips are pivotally attached to two bars, one of the strips being attached to both bars and the other two strips being attached each to one of the bars. The trip bars are so arranged that when displaced due to deflection of the strips a trip mechanism is operated in such a manner as to deenergize or energize the control system of a contactor or circuit breaker and cause such contactor or circuit breaker to open.

The arrangement of the trip bars is such that when the thermostatic strips deflect evenly the trip bars are lifted together and operate the trip mechanism. If, however, the deflection is unequal, as will occur when one phase is carrying no current, one or both bars will be tilted and will trip the circuit with a lower actual deflection of the active strips than that necessary to cause tripping under conditions of even heating.

The invention will be better understood by reference to the accompanying diagrammatic drawings which illustrate one embodiment, Fig. 1 being a plan view of the device, Fig. 2 a front elevation and Figs. 3, 4 and 5 show the efiect of various phases being deenerglzed.

It will be understood that all constructional features well known to those skilled in the art are omitted to bring out more clearly the essential features of the invention.

Referring to Fig. 1, 8 is a base on which are mounted three thermostatic strips la, lb and lo. Below these strips are mounted three heaters 2a, 2b and 20 which are adapted to be connected in each of the three phases of a three phase circuit. Pivotally connected to la. and lb is the trip bar 4 and pivotally connected to lb and la is the trip bar 3. The tripping mechanism 5 cooperates with trip bars 3 and 4 to raise moving contact 6 out of contact with fixed contact I. It will be appreciated that whereas a slow break contact mechanism 6 and l is illustrated for the sake of simplicity, a quick break contact or a latched contact could be substituted without departing from the scope of the invention.

to current sensitive de- "vices, more particularly to overload protective 4 Claims. (01. 200- 122) If equal currents are passed through the three heaters, the heaters will attain equal temperatures and the three thermostatic strips l 0, lb and lo will be equally deflected. Under this condition the trip bars 3 and 4 will be raised together, so that they will remain approximately parallel until the temperature is raised to such a. point that the tripping mechanism 5 is actuated. The movement of the bars will be equal to the deflection of the ends of the strip.

Figs. 3, 4 and 5 illustrate what will occur when open phasing takes place and various heaters are deenergized.

Fig. 3 illustrates the condition when strips la and lb are deflected due to the heat generated by their respective heaters whereas strip la is not deflected since no current passes through the heater due to the phase being open circuited. Trip bar 4 is parallel with its position at rest and has moved through a distance equal to the deflection Of strips la and lb. Trip bar 3 is tilted and the operative point which engages with trip mechanism 5 has moved through a distance equal to the movement of strip l b multiplied by the length of the lever from pivotal attachment to strip lc to point of application to 5 and divided by the length of the lever from its pivotal attachment to lc to its pivotal attachment to lb.

Fig. 4 illustrates exactly the same condition when strip la is unheated. In this case, trip bar 4 is tilted.

Fig. 5 illustrates the condition which obtains when strip lb is unheated. In this case, both trip bars 3 and 4 are tilted.

In these drawings, the bimetal thermostatic strips are shown as flat, but according to design requirement, these may be U-shaped, spiral or helical without in any way departing from the spirit and scope of this invention. In the same way the form and shape of the heaters may be varied, and these may either carry the whole of the phase current, or may be shunted by noninductive resistances, or may be in the secondary circuit of current transformers.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electric overload protective device for a polyphase circuit comprising a pair of switch contacts, a pair of movable current responsive elements each associated with one phase of said circuit, said elements being arranged for displacement in parallel spaced relation and each in an amount proportional to the current in the associated phase, an actuating lever disposed transversely of the direction of movement of said elements and pivotally mounted upon each of said elements at spaced apart points intermediate its ends, and means operatively connecting at least one end of said lever to actuate said switch contacts only upon the occurrence of excessive current in one or more of said phases.

2. An electric overload protective device for a polyphase circuit comprising a pair of switch contacts, a pair of thermal responsive elements movable in parallel spaced relation and each arranged to be heated by the current in a separate phase of said circuit, an actuating le'ver disposed transversely of the direction of movement of said thermal responsive elements and pivotally mounted upon each of said elements at spaced apart points intermediate its ends, and means operatively connecting at least one end of said lever to actuate said switch contact only upon the occurrence of excessive current in one or more of said phases.

3. An electric overloadprotective device for a three phase circuit comprising a pair of switch contacts, three thermal responsive members arranged for deflection in the same direction and in parallel spaced relation, each said thermal responsive member being arranged to be heated in accordance with the current in a separate phase of said circuit, a pair of actuating levers disposed transversely of the direction of deflection of said thermal responsive members, each said lever being 'pivotally connected to a common one of said thermal responsive members and each of the other thermal responsive members being 'pivotally con- 4 nected to a separate one of said levers, and means operatively connecting at least one end of each of said actuating levers to separately actuate said contacts only upon the occurrence of excessive current in one or more of said phases. 4. An electric overload protective device for a three phase circuit comprising a pair of switch contacts, three thermal responsive members arranged for deflection in the same direction and in parallel spaced relation, means for heating each of said thermal responsive members in accordance with the current in a separate phase of said circuit, a pair of actuating levers disposed transversely of the direction of deflection of said thermal responsive members, each said lever being pivotally connected at its center to the central one of said thermal responsive members and being pivotally connected at a point intermediate its ends to a different one of the two outside thermal responsive members, and means operatively connecting each end of each of said actuating levers to separately actuate said switch contacts upon the occurrence of an excessive current in one or more of said phases.

ERIC H. B. MARTIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,199,437 .B'esag May '7, 1940 

